Generally, a semiconductor light-emitting device has a light-emitting diode (LED) and a laser diode (LD) generating light when a forward current flows. Particularly, the LED and LD have a common p-n junction, and when a current is applied to the light-emitting device, the current is converted to photons and thereby light is emitted from the device. The light emitted from the LED and LD has various wavelengths from a long wavelength to a short wavelength depending on the semiconductor material(s). Above all, LEDs made from wide band-gap semiconductors allow red, green and blue colors in visible bands and have been applied widely in industries such as displays for electronic devices, traffic lights, and various light sources for display devices. Due to the development of white light in recent years, it will be widely used as the next generation light source for general lighting.
A Group III-V nitride-based semiconductor is generally grown hetro-epitaxially on the upper part of sapphire, silicon carbide (SiC), or silicon (Si) which is an initial substrate having a significantly different lattice constant and thermal expansion coefficient to obtain high quality semiconductor thin films. However, since the sapphire initial substrate has poor thermal conductivity, it cannot apply a large current to LEDs. Since the sapphire initial substrate is an electrical insulator and thereby is difficult to respond to static electricity flowed in from outside, it has a high possibility to cause failure due to the static electricity. Such drawbacks not only reduce reliability of devices but also cause a lot of constraints in packaging processes.
Further, the sapphire initial substrate, which is an insulator, has a MESA structure in which both an n-type ohmic contact electrode (hereinafter referred as to “first ohmic contact electrode”) and a p-type ohmic contact electrode (hereinafter referred as to “second ohmic contact electrode”) are formed in the same growth direction as that of a multi-layered light-emitting structure. Since an LED chip area should be higher than a certain size, there is limit to reducing the LED chip area, restricting the improvement of LED chip production.
In addition to these disadvantages of the MESA-structured LEDs grown on the upper part of the sapphire substrate as an initial substrate, it is difficult to release a great amount of heat outward generated inevitably during the operation of the light-emitting device since the sapphire substrate has poor thermal conductivity. Due to these reasons, there is a limitation in applying the MESA structure, to which the sapphire substrate is attached, to light-emitting devices used for a large area and a large capacity (that is, a large current) such as the light for large displays and general lighting. When a high current is applied to a light-emitting device for a long period of time, the internal temperature of a light-emitting active layer is gradually increased largely due to the generated heat and thereby an LED light-emitting efficiency is gradually decreased.
A silicon carbide (SiC) substrate, unlike the sapphire substrate, not only has good thermal and electric conductivity but also allows a multi-layered light-emitting structure thin film to be laminated and grown since it has a similar lattice constant and thermal expansion coefficient (TEC), which are important factors in the semiconductor single crystal thin film growth, as that of Group III-V nitride-based semiconductors. Further, it allows the manufacturing of various types of vertical-structured light-emitting devices. However, because producing a high quality SiC substrate is not easy, it is more expensive than producing other single crystal substrates, making it difficult for mass production.
Therefore, it is most desirable to provide a high-performance light-emitting device by using a multi-layered light-emitting structure laminated and grown on a sapphire substrate in view of the technology, economy and performance. As described above, much effort has been made to produce a high-performance vertical structured LED by growing a high quality multi-layered light-emitting structure thin-film on the upper part of a sapphire initial substrate, lifting-off the Group III-V nitride-based semiconductor multi-layered light-emitting structure thin film from the sapphire substrate and using the result, in order to resolve the problems associated with the MESA-structured LEDs produced by using a thin film which is Group III-V nitride-based semiconductor multi-layered light-emitting structure laminated/grown on the upper part of a sapphire substrate which is an initial substrate.
FIG. 1 is a sectional view illustrating a process for separating a sapphire initial substrate by employing a conventional laser lift off (LLO) process. As shown in FIG. 1, when a laser beam, which is a strong energy source, is irradiated to the backside of a transparent sapphire initial substrate 100, the laser beam is absorbed strongly at the interface and the temperature of 900° C. or higher is thereby generated momentarily and causes thermochemical dissociation of gallium nitride (GaN) at the interface, and further separates the sapphire initial substrate 100 from the nitride-based semiconductor thin film 120. However, it has been reported in many documents that in the laser lift-off process of the Group III-V nitride-based semiconductor multi-layered light-emitting structure thin film, the semiconductor single crystal thin film is damaged and broken after being separated from the sapphire substrate due to a mechanical stress generated between the thick sapphire initial substrate and the Group III-V nitride-based semiconductor thin film because of the difference in the lattice constant and thermal expansion coefficient. When the Group III-V nitride-based semiconductor multi-layered light-emitting structure thin film is damaged and broken, it causes a large leaky current, reduces the chip yield of light-emitting devices and reduces the overall performance of the light-emitting devices. Therefore, studies are currently under way for manufacturing a high-performance vertical-structured LED by using the lift-off process of the sapphire substrate which can minimize damage to the Group III-V nitride-based semiconductor multi-layered light-emitting structure thin film and the separated semiconductor single crystal thin film.
Various methods have been suggested to minimize damage and breaking of the Group III-V nitride-based semiconductor multi-layered light-emitting structure thin film when the sapphire initial substrate is separated by the LLO process. FIG. 2 is sectional views illustrating a process for forming a stiffening supporting substrate in the growth direction by employing a wafer bonding, electro plating or electroless plating process prior to the LLO process according to a conventional technology to prevent damage and breaking of a semiconductor multi-layered light-emitting structure thin film. Referring to (a) in FIG. 2, a supporting substrate 240, which is strongly adhered and is structurally stable by using wafer bonding, is formed on the upper part of a bonding layer 230 before lifting off semiconductor single crystal multi-layered light-emitting structure thin films 210, 220 from an initial substrate 200 by irradiating the backside of the initial substrate made of transparent sapphire with a laser beam. Referring to (b) in FIG. 2, a supporting substrate 242, which is strongly adhered and is structurally stable, is formed on the upper part of a seed layer 232 by using an electro plating process before lifting off the semiconductor single crystal multi-layered light-emitting structure thin films 210, 220 from the initial substrate 200 made of sapphire.
FIG. 3 is a sectional view illustrating vertical-structured Group III-V nitride-based semiconductor light-emitting devices manufactured by introducing the supporting substrate, which is strongly adhered and is structurally stable, according to the conventional technology used in the process of FIG. 2.
The figure indicated by (a) in FIG. 3 is a sectional view illustrating a semiconductor light-emitting device manufactured by using the method for manufacturing the supporting substrate indicated by (a) in FIG. 2. Referring to (a) in FIG. 2 illustrating an LED section bonded with a wafer, it is successively constituted with a supporting substrate 340, which is a thermal and electrical conductor, a bonding layer 330, a multi-layered metal layer 350 including a second ohmic contact electrode, a second semiconductor cladding layer 380, a light-emitting active layer 370, a first semiconductor cladding layer 360, and a first ohmic contact electrode 390. A semiconductor wafer such as silicon (Si), germanium (Ge), silicon-germanium (SiGe), gallium arsenide (GaAs) and the like having an excellent electrical conductivity is preferably used as the electro conductive supporting substrate 340.
However, the supporting substrate 340, used for the vertical-structured light-emitting device (LED) as shown in (a) of FIG. 3, causes significant wafer warpage and fine micro-cracks inside the semiconductor multi-layered light-emitting structure when Si or another conductive supporting substrate wafer is bonded by wafer bonding because it has a significant difference in thermal expansion coefficient (TEC) against the sapphire substrate on which the semiconductor single crystal thin film is grown/laminated. Such problems further cause processing difficulties and lower the performance of LED manufactured therefrom and the product yield.
The figure indicated by (b) in FIG. 3 is a sectional view illustrating a semiconductor light-emitting device manufactured by using the method for manufacturing the supporting substrate indicated by (b) in FIG. Referring to (b) in FIG. 3 illustrating the sectional view of the LED formed through electro plating, the vertical-structured light-emitting device (LED) formed through an LLO and electro plating process is successively constituted with a supporting substrate 342, which is electrically conductive, a seed layer 332, a multi-layered metal layer 352 including a second ohmic contact electrode, a second semiconductor cladding layer 380, a light-emitting active layer 370, a first semiconductor cladding layer 360, and a first ohmic contact electrode 390. The electrically conductive supporting substrate 342, which is a metallic thick film formed through electro plating, is preferably formed with a single metal such as Cu, Ni, W, Au, Mo and the like or an alloy composed thereof.
The LED supporting substrate 342 having the structure described above as shown in (b) of FIG. 3 has a significantly higher thermal expansion coefficient and flexibility than the sapphire substrate due to the metal or alloy thick film formed through electro plating, thereby causing curling, warpage, breaking, etc.
Therefore, it is highly demanded that highly efficient supporting substrates and methods for manufacturing the high performance vertical-structured light-emitting devices using the same are develop to resolve the problems of wafer warpage, breaking, micro-crack, annealing and singulate chip processing, post-processing problems, low product yield, etc. while manufacturing the vertical-structured Group III-V nitride-based semiconductor light-emitting device using the LLO process.